Selective silicon nitride plasma etching process

ABSTRACT

A two step method of etching a silicon nitride layer carrying a surface oxygen film from a substrate in a plasma reactor employs the steps of (1) a breakthrough step of employing a plasma of oxygen free etchant gases to break through and to remove the surface oxygen containing film from the surface of the silicon nitride layer, and (2) a main step of etching the newly exposed silicon nitride with etchant gases having high selectivity with respect to the silicon oxide underlying the silicon nitride. The plasma etching can be performed while employing magnetic- enhancement of the etching. The plasma etching is performed in a plasma reactor comprising a low pressure, single wafer tool. Plasma etching is performed while employing magnetic-enhancement of the etching. The etchant gases include a halide such as a bromide and a fluoride in the breakthrough step. The etchant gases include an oxygen and bromine containing gas in the main step.

This is a continuation application of application Ser. No. 07/700,871filed May 9, 1991, now U.S. Pat. No. 5,188,704.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to plasma etching of silicon nitride.

2. Discussion of Related Art

One selective nitride etching process for producing an sidewall processutilizes a Cl₂ /O₂ /Ar plasma etching chemistry at a selectivity ofnitride over oxide of about 4-5:1. Overetching of such a silicon nitridelayer can lead to total loss of an underlying thin SiO₂ stopping layerand there is the potential of damage to the underlying Si substrate atthe surface juxtaposed with the emitter opening in the surface of the Sisubstrate of a transistor being formed in the Si substrate.

U.S. Pat. No. 4,374,698 of Sanders et al, for "Method of Manufacturing aSemiconductor Device" etches silicon nitride and silicon oxide in aplasma with the silicon nitride layers being etched five times fasterthan the silicon oxide layers. The use of a gaseous compound containinga halogen other than a fluoride is described.

The gas composition preferably includes

    ______________________________________                                        62.5%           SiF.sub.4                                                     A halogen compound other than a fluoride:                                     7.5% (2-8% pref)                                                                              CF.sub.3 Br or                                                1-15%           CF.sub.2 Cl2                                                  An oxidation compound or molecule:                                            30%             NO (range 20-40%) or                                          3-10%           O.sub.2                                                       0%              diluent gas listed                                            Preferred reactor conditions                                                  pressure        100 Pascal                                                    temperature     125 deg. C. substrate temp.                                   RF power        150 Watts                                                     ______________________________________                                    

U.S. Pat. No. 4,793,897 of Dunfield et al for "Selective Film EtchProcess" uses " . . . a reactant gas mixture of fluorinated etching gasand oxygen for selectively etching a thin film of material such assilicon nitride with high selectivity for a silicon oxide underlayer . .. " without any magnetic field and without any other halides than SiF₄.

The gas composition preferably includes

    ______________________________________                                        10-400 sccm         Total gas flow                                             0-100 sccm         NF.sub.3                                                   0-100 sccm         SiF.sub.4                                                  0-100 sccm         O.sub.2                                                    0-100 sccm         He                                                        10-150 sccm         Chamber                                                   Preferred reactor conditions                                                  pressure            0.5-30 mTorr                                              temperature         25 degrees C.                                             RF power            100-1500 watts                                            RF Frequency        none listed                                               ______________________________________                                    

U.S. Pat. No. 4,844,773 of Loewenstein et al for "Process for EtchingsSilicon Nitride Film" with respect to FIG. 32 and the process unit 1300describes a process adapted for a low pressure silicon nitride etch. Itis stated that "HBr or CF₃ Br provides a very potent passivatingchemistry for fluorine-based etches." The Loewenstein patent suggestsuse of SiF₄ with HBr to etch a thin film of tungsten. However, withrespect to silicon nitride, the examples of etches are as follows:

EXAMPLE 1

Gas composition preferably includes

    ______________________________________                                        1000 sccm        Helium                                                        200 sccm        CF.sub.4 (F.sub.2, CHF.sub.3, C2F.sub.6, SF.sub.6,                            F.sub.3, or combinations with CF.sub.4)                      Preferred reactor conditions                                                  pressure         0.7 Torr                                                     temperature      25 degrees C.                                                RF power         225 watts                                                    RF Frequency     13.56 MHz                                                    Remote RF power  400 watts                                                    Remote RF Frequency                                                                            2450.00 MHz                                                  ______________________________________                                    

EXAMPLE 2

Gas composition preferably includes

    ______________________________________                                        500 sccm             Helium                                                   100 sccm             SF.sub.6 (or F.sub.2, CF.sub.4,                                               or C2F.sub.6)                                            Preferred reactor conditions                                                  pressure             50 mTorr,                                                temperature          25 degrees C.                                            RF power             200-300 watts                                            RF Frequency         13.56 MHz                                                Remote RF power      400 watts                                                Remote RF Frequency  2450.00 MHz                                              ______________________________________                                    

The Loewenstein et al patent mentions a magnetron shown in FIG. 15 ofthat patent in connection with a remote plasma chamber as "an example ofa structure which generates activated species by gas flows through aplasma discharge which is remote from the wafer face . . . " There is nomention of the application of a magnetic field to a wafer directly.Instead, a remote gas plasma mixture is applied to the work. There is nodiscussion of a magnetically confined plasma, but only the remotemagnetron is discussed.

It, is an object of this invention to provide a new etching process,capable of etching silicon nitride over SiO₂ very selectively forproducts where the semiconductor device structure would have an eventhinner SiO₂ stopping layer during silicon nitride etching where it isrequired to assure product manufacturability and acceptable yieldcapacity in manufacturing.

In accordance with-this invention silicon nitride is etched with aselectivity with respect to SiO₂ better than 6:1 up to over 100:1depending upon the gas composition and the process conditions employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a chamber housing a plasma etching reactor with anelectromagnetic unit for enhancing the plasma.

FIG. 2 and FIG. 3 are graphs of etch rates and nitride to oxideselectivity respectively as a function flow rates of helium and oxygen.

FIG. 4 is a graph of etch rate as a function of flow rates of helium andoxygen with SiF₄ addition.

FIG. 5 is a graph of nitride/oxide selectivity as a function of heliumoxygen flow rates with SiF₄ addition.

FIG. 6 is a sectional view of a substrate having an opening with a layerof silicon dioxide at the base. The substrate is prepared for processingwith silicon nitride.

FIG. 7 is a view of the product of FIG. 6 after a layer of siliconnitride has been deposited.

FIG. 8 is a view of the product of FIG. 7 after a layer of silicondioxide has been deposited on the layer of silicon nitride.

FIG. 9 is a view of the product of FIG. 8 after the top layer of silicondioxide has been etched away leaving a sidewall structure of silicondioxide on the sides of the opening in the substrate.

FIG. 10 is a view of the product of FIG. 9 after the silicon nitrideexposed has been etched away between the sidewall structures.

DESCRIPTION OF THE PREFERRED EMBODIMENT

1. Process Overview

This invention utilizes a two-step process in a low pressure, singlewafer, plasma reactor employing magnetic enhancement to perform thesteps as follows:

(1) break through and to remove surface oxide or oxynitride from thesurface of a top silicon nitride layer, and

(2) to etch the newly exposed silicon nitride with high selectivity withrespect to the silicon oxide underlying the silicon nitride.

The first step employs fluorine radicals to etch away the surface oxygenimpurities from the silicon nitride layer in an oxygen-free plasma.Then, the second step uses bromine-containing and oxygen-containinggases to etch primarily the exposed surfaces of the silicon nitridelayer in a plasma etching atmosphere which is also magneticallyenhanced.

The preferred reactive gas for the step of breaking through theoxynitride layer is SF₆ gas with or without a small amount of HBr gas asan additive. The silicon nitride surface oxide or oxynitride impurities,normally less than 50 Angstroms in thickness can be removed from thework piece in a gaseous plasma within a few seconds. The preferred mainetchant for silicon nitride etching is a mixture of HBr and O₂, with orwithout diluent such as He. HBr dissociates readily in the presence ofoxygen in the plasma and its dissociation can be enhanced by theapplication of a rotational magnetic field. (See reaction equation (1)below.) The bromine radical is the main etchant for silicon nitride.(See reaction equation (2) below.) At the point of depletion of siliconnitride, an equilibrium exists at the nitride-oxide interface where therate of oxide etching (forward reaction) is competing with there-oxidation of silicon bromide or redeposition of oxide (reversedreaction, (see reaction equation (3) below). This equilibrium results ina substantially lower rate of etching of the underlying oxide whichfavors a high selectivity of silicon nitride-to-silicon oxide in theoverall reaction. ##STR1##

Silicon-containing gases such a silicon tetrafluoride can be used as anadditive agent to increase the deposition of silicon oxide at thesubstrate surface, thus further depressing the etching of silicon oxidein reaction (3) and resulting in an even higher degree of selectivity.However, the addition of silicon tetrafluoride also reduces the etchingrate of silicon nitride, which can have a negative impact upon waferprocessing throughput in semiconductor device manufacturing.

Plasma Processing System

FIGS. 1A and 1B show a plasma processing apparatus suitable for use forperforming the processes of this invention. A plasma reactor 20 includeswalls 21 housing a reactor chamber 22 wherein a plasma of neutral (n)particles, positive (+) paticles, and negative (-) particles are found.Walls 21 include cylindrical wall 44 and cover 46. Plasma processinggases are introduced via inlets 24 into reactor chamber 22. Plasmaetching gases are introduced into chamber 22 through inlets 24. A watercooled cathode 26 is connected to an RF power supply, 28 at 13.56 MHz.An anode 29 is connected to the walls 21 which are grounded by line 30.Helium gas is supplied through passageway 40 through cathode 26 to thespace beneath wafer 38 which is supported peripherally by lip seal ring42 so that the helium gas cools the wafer 38. Wafer support 36 includesa plurality of clamps not shown which hold down the upper surface ofwafer 38 at its periphery as is well known to those skilled in the art.A pair of helmholtz configured electromagnetic coils 32 and 33 providingnorth and south poles within the chamber 22 are disposed at oppositeends of the lateral cylindrical wall 44 of the walls 21. The coils 32and 33 provide a transverse magnetic field with the north and southpoles at the left and right providing a horizontal magnetic field axisparallel to the surface of the wafer 38. The transverse magnetic fieldis applied to slow the vertical velocity of the electrons which areaccelerated radially by the magnetic field as they move towards thewafer. Accordingly, the quantity of electrons in the plasma is increasedby means of the transverse magnetic field and the plasma is enhanced asdescribed in Foster et al U.S. Pat. No. 4,668,365 for "MagnetronEnhanced Plasma Etching Process", Wong et al European Patent ApplicationNo. EP0 272 143-A for "Bromine and Iodine Etch Process for Silicon andSilicides", and Andrews et al European Patent Application No. EP-0 272142-A which relates to a magnetically enhanced plasma etch reactor forsemiconductor wafers which provides a magnetically controlled magneticfield for etching. Electromagnets which provide the magnetic field areindependently controlled to produce a field intensity orientation whichis uniform. The field can be stepped angularly around the wafer byrotating the energization of the electromagnets, sequentially. U.S. Pat.No. 4,740,268 of Bukhman describes another "Magnetically Enhanced PlasmaSystem" which rotates a transverse magnetic flux field B over a wafer,parallel to its surface, in an electrically excited plasma chamber. Thelines of flux are normal to the electric field E which is directednormal to the wafer of the lower electrode in the Bukhman patent.

2. Magnetic Field Enhancement

The use of a transverse magnetic field directed parallel to the surfacebeing treated by the plasma and the cathode of the plasma reactorincreases ionization efficiency of the electrons in the plasma. Thisprovides the ability to decrease the potential drop across the cathodesheath and to increase the ion current flux present on the wafersurface, thereby permitting higher rates of etching without requiringhigher ion energies to achieve the result otherwise.

The preferred magnetic source used to achieve magnetically enhanced RIEused in practicing this invention is a variable rotational fieldprovided by two sets of electromagnetic coils arranged in a Helmholtzconfiguration. The coils are driven by 3-phase AC currents. The magneticfield with flux B is parallel to the wafer substrate, and perpendicularto the electrical field as shown in FIG. 1A. Referring to FIG. 1B, thevector of the magnetic field H which produces flux B is rotating aroundthe center axis of the electrical field by varying the phases of currentflowing through coils at a typical rotational frequency of 0.01 to 1 Hz,particularly at 0.5 Hz. The variable strength of the magnetic flux Btypically from 0-150 Gauss is determined by the quantities of thecurrents supplied to the coils.

In the case of etching of silicon nitride with an etchant of gasesincluding bromine and oxygen, the magnetic field flux, typically at30-60 Gauss provides enhancement to the dissociation of the Br compounds(reaction (1)) and also O₂ gas, and thus increases the rate of etchingof silicon nitride. On the other hand, the etching-reoxidationequilibrium of reaction (3) is not being affected. Thus, the higherselectivity of silicon nitride to the underlying silicon oxide isobtained.

3. Process Applications and Trends

Highly selective etching of silicon nitride over underlying siliconoxide can be achieved by taking advantage of the magnetically enhancedplasma in the reactor system of FIG. 1A using gases containing bromineand oxygen. The flow rates of oxidant etchant such as O₂ control therate of etching and the selectivity of nitride/oxide as shown in FIGS. 2and 3. At these selected conditions, nitride etching rates are not astrong function of the percent of oxygen in the total gas flow. However,the oxide etching rate is initially decreased as small amounts of-oxygenare included in the gas mixture, and the oxide etching rate levels offas the oxygen flow rate is increased further. The helium in the etchantgases is used as a reaction inert material or as a diluent, which doesnot affect the etching rate or the selectivity of the etching process.The different process trends for nitride and oxide rates of etching withrespect to increasing oxygen composition in the total gas flow allow theselectivity of nitride to oxide to increase an to reach an optimum ataround 12.6:1, as indicated in FIG. 3. Referring to reaction equation(3) above, increasing the flow rate of oxygen gas can result in anincrease in the rate of re-oxidation of SiBr_(y) species and can formdeposited SiOx ⁻ r SiO₂, which can cause a reduction of the etching ratewhich can cause a corresponding increase of relative etching selectivityof nitride-to-oxide. However, a further increase of the flow rate ofoxygen gas can result in excess SiO₂ formation which hampers the rate ofetching of the nitride. In general, an optimal rate of etching and anoptimal selectivity ratio are both dependent upon an appropriatecomposition of the O₂ etchant in the total gas flowing through thechamber.

FIG. 4 indicates the effect of adding a silicon containing gas,particularly SiF₄, to the reactive gas. Addition of SiF₄ can have aneffect on rate of etching for selected process conditions. As indicatedby FIG. 4, both the rates of nitride and oxide etching were moresignificantly influenced by the increase of the oxygen flow rate in thepresence of 1.5 sccm (standard cubic cm per minute) of SiF₄ in the totalgas flow. It is envisioned as the oxygen concentration increases thatthe rake of SiO₂ deposition increases sharply as a result of thefollowing reaction: ##STR2## Both nitride and oxide rates of etchinghave shown a downward trend as the SiO₂ deposition increases with theincreasing of the oxygen flow rate. The nitride-to-oxide selectivity iscalculated from the nitride etching rate divided by the oxide etchingrate, which increases sharply with increasing oxygen flow and it reachesinfinity at a zero oxide rate as shown in FIG. 5. Further increase inthe oxygen flow rate results in a net SiO₂ deposition at thenitride/oxide interface.

FIG. 1A AND FIG. 1B shows a chamber housing a plasma etching reactorwith an electromagnetic unit for enhancing the plasma.

Method 1

We have discovered when etching away films of silicon nitride withfluoride containing etchants in the presence of oxygen that a film ofthin oxide or oxynitride prevents rapid etching of the silicon nitridefilm. However, the use of oxygen is helpful for the purpose of etchingaway the silicon nitride once the oxynitride film has been removed fromthe surface of the silicon nitride layer. Accordingly we have developeda two step process of plasma etching silicon nitride selectively withrespect to other films such as silicon oxide or silicon dioxide.

PROCESS I

Breakthrough of the Surface of Oxynitride

The first phase of the process is to break through, i.e. clean and etchaway the surface oxynitride which is a thin layer of about 50 Angstromsin thickness on the surface of the layer of silicon nitride. The firstphase is performed in the absence of an oxidant such as oxygen, sincethe purpose of this step is to reduce the oxygen which forms theoxynitride layer. In the case of both Process I and Process II below, areactive plasma is formed in the reactor including HBr and a gas fromthe group of fluorine-containing gases consisting essentially of SF₆,CF₄ and C2F₆ and NF₃. The gas composition preferably includes

    ______________________________________                                        (0-90%)            HBr                                                        (10-100%)          fluorine-containing gas.                                   (0%)               oxygen or oxidant gas.                                     Preferred reactor conditions                                                  pressure           low                                                        Temperature        25 degrees C.                                              RF power           200-300 watts                                              rotational magnetic field                                                                        0-45 Gauss.                                                ______________________________________                                    

Main Etching Step: Plasma Etching of Si₃ N₄

In the second phase of the process, with the oxynitride removed, theexposed layer of Si₃ N₄ is etched with a bromine and oxygen containingreactive gas mixture preferably consisting of HBr, an oxidant selectedfrom the group consisting of O₂, CO₂, or N₂ O and a diluent gas such asHe, N₂ or Ar. It will be noted that in this case the main etching stepis performed in the absence of SiF₄ or any other fluorine containinggases. The preferred reactor conditions are as follows:

TABLE I

The gas composition broadly includes as follows:

    ______________________________________                                        60-95%           HBr                                                           0%              SiF.sub.4                                                     2-15%           an oxidant (O.sub.2, CO.sub.2, or N.sub.2 O)                  0-45%           diluent gas                                                  Preferred reactor conditions                                                  pressure         50-150 mTorr                                                 RF power         200-400 watts                                                rotational magnetic field                                                                      25-70 Gauss                                                  Si.sub.3 N.sub.4 etch rate                                                                     300-1000 Angstroms/min                                       ______________________________________                                    

The selectivity of Si₃ N₄ /SiO₂ is better than 6:1 up to 12.6:1 in ourexperiments.

More generally, the process parameters fall into ranges as followslisted on the basis of flow rates of the gases as listed in TABLE IIbelow.

                  TABLE II                                                        ______________________________________                                        Process    Broad       Preferred   Optimum                                    ______________________________________                                        Gas Flow, sccm                                                                HBr        0-20 (0-91%)                                                                              5-15 (45-83%)                                                                             10 (77%)                                   SF.sub.6   2-10 (9-100%)                                                                             3-6 (17-55%)                                                                              3 (23%)                                    Pressure, mT                                                                             20-150      50-100      50                                         Power Density                                                                            0.82-3.3    1.63-2.4    1.65                                       (W/cm2)                                                                       Time, sec  4-20        6-10        8                                          Magnetic Field                                                                           0-45        0-25        0                                          Gauss                                                                         MAIN ETCH                                                                     Gas Flow, sccm                                                                HBr        10-30 (71-96%                                                                             15-25 (80-95%)                                                                            20.00 (93%)                                O.sub.2    0.15-2 (2-7%)                                                                             0.3-1 (2-5.7%)                                                                            0.45 (2.1%)                                He         0-10 (0-25%)                                                                              0-2 (0-20%) 1.05 (4.9%)                                Pressure, mT                                                                             20-150      50-120      100                                        Power Density                                                                            0.82-3.3    1.63-2.4    1.65                                       (W/cm2)                                                                       Etch Rate  300-1000    500-700     600                                        (A/min)                                                                       Magnetic Field                                                                           0-75        25-60       45                                         Gauss                                                                         Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                    LPCVD Nitride                                                                            >4:1        >6:1        12.6:1                                     Plasma Nitride                                                                           >4:1        >6:1        13.5:                                      ______________________________________                                    

PROCESS II

Plasma etching the Si₃ N₄ layer over the SiO₂ layer in amagnetically-enhanced etch reactor with the steps as follows.

Breakthrough of the Surface of Oxynitride

This step is the same as in proposed process I.

Main Etching Step: Plasma Etching of Si₃ N₄

In the second phase of process II, with the oxynitride removed, the nowexposed layer Si₃ N₄ is etched with a bromine, oxygen and fluorinecontaining reactive gas mixture preferably consisting of HBr, SiF₄, anoxidant selected from the group consisting of O₂, CO₂, or N₂ O and adiluent gas selected from the group He, N₂ and Ar. The preferred reactorconditions are as follows:

The gas composition broadly can include as follows:

    ______________________________________                                        50-83%           HBr                                                           5-15%           SiF.sub.4                                                     2-15%           an oxidant (O.sub.2, CO.sub.2, or N.sub.2 O)                  0-45%           diluent gas                                                  Preferred reactor conditions                                                  pressure         50-150 mTorr,                                                RF power         200-400 watts                                                rotational magnetic field                                                                      25-70 Gauss.                                                 Si.sub.3 N.sub.4 etch rate                                                                     300-800 A/min                                                ______________________________________                                    

Using the process II etching of nitride occurs with a selectivity of Si₃N₄ /SiO₂ of better than 6:1 and up to over 100:1. However, a loss ofnitride etch rate an an increase in etch non-uniformity are accompaniedwith the increased selectivity.

The data listed in TABLE IV below illustrates the useful ranges ofprocess parameters for selectively etching the nitride over underlyingthermal oxide or plasma oxide. The selectivity for the plasma depositednitride is slightly higher than that for LPCVD (Low Pressure ChemicalVapor Deposition) nitride. The application of magnetic field enhancementto the plasma is essential for providing high ionization efficiency andadequate etching rate of the nitride layer. Also as shown in Table II,the addition of SiF₄ in the HBr/O₂ /He etchant is the main etching stepwhich enhances the rate of SiO₂ deposition and increases nitrideto-oxide selectivity.

                  TABLE IV                                                        ______________________________________                                        Process    Broad       Preferred   Optimum                                    ______________________________________                                        Gas Flow, sccm                                                                HBr        0-20        5-15        10                                         SF.sub.6   2-10        3-6         3                                          Pressure, mT                                                                             20-150      50-100      50                                         Power Density                                                                            0.82-3.3    1.63-2.4    1.65                                       (W/cm2)                                                                       Time, sec  4-20        6-10        8                                          Magnetic Field                                                                           0-45        0-25        0                                          Gauss                                                                         MAIN ETCH                                                                     Gas Flow, sccm                                                                HBr        10-30 (50-90%)                                                                            10-20 (70-85%)                                                                            15 (83%)                                   O.sub.2    0.15-2 (2-15%)                                                                            0.3-1 (3-7%)                                                                              .45 (2.6%)                                 He         0-10 (0-30%)                                                                              0-4 (0-20%) 1.05 (6.0%)                                SiF.sub.4  0.5-2.5 (5-15%)                                                                           1-2 (5-9%)  1.5 (8.4%)                                 Pressure, mT                                                                             20-150      50-120      100                                        Power Density                                                                            0.82-3.3    1.63-2.4    1.65                                       (W/cm2)                                                                       Etch Rate  200-700     250-450     300                                        (A/min)                                                                       Magnetic Field                                                                           0-75        25-60       45                                         Gauss                                                                         Si.sub.3 N.sub.4 -to-                                                                    >6:1        >10:1       17:1                                       SiO.sub.2 selectivity                                                         LPCVD Nitride                                                                 ______________________________________                                    

EXAMPLE

Referring to FIGS. 6-10, the drawings show sectional views of theresults of a sequence of steps in an integrated advanced bipolarsemiconductor sidewall process. Referring to FIG. 6, a wafer with asilicon substrate 10, is coated with a film 11 of polysilicon which hasa contact opening 12 with a lower layer of SiO₂ 14 at the bottom ofopening 12. The process includes steps as follows:

1) FIG. 7 shows a deposit on the wafer of FIG. 6 of a thin film 16 ofSi₃ N₄ (approx. 200-500 Angstroms thick) over the film 11 and over thelayer of SiO₂ 14 at the base of opening 12. The opening 12 and sidewalland the polysilicon film 11 are all coated with the silicon nitride film16.

2) Referring to FIG. 8, using a TEOS (TetraEthylOrthoSilicate) liquid,deposit SiO₂ (approx. 1000-3000 Angstroms thick) on top of the nitridelayer produced in step 1) above. This produces an SiO₂ sidewall layer 18on top of thin film 16.

3) Referring to FIG. 9, an etch of the top SiO₂ layer 18 has beenperformed to leave just the sidewalls 18' as the lower layer of Si₃ N₄16 is exposed by the removal of the remainder of layer 18. A CF₄ /Arplasma forms the SiO₂ sidewalls 18' at the opening with a 15% overetch.The endpoint is controlled by an optical emission monitoring device.

4) In FIG. 10 is shown the device of FIG. 9 after etching away of theexposed surface of the underlying Si₃ N₄ layer 16 in hole 12, exposingthe surface of layer 14 of SiO₂ from FIG. 6, which is carrying thesidewall pattern 18', as well as, the edges of layer 16. The etchantused is HBr/SF₆ and HBr/O₂ /He according to the process conditions inProcess I above. The process I steps is performed by a time-etch of 60sec. which includes a 25% overetch. Measurements have indicated acomplete removal of nitride over SiO₂ and no measurable loss of the SiO₂in the sidewall pattern 18' during that etch.

What is claimed is:
 1. A method of etching a silicon nitride layer on asubstrate, said substrate coated on its surface with a silicon oxidelayer, in turn coated on its surface with a silicon nitride layer, inturn coated on its surface with an oxide or oxynitride containing film,said method comprising performing in a plasma reactor the steps on saidsubstrate, as follows:(1) a breakthrough step of employing a plasma ofoxygen free etchant gases to break through and to remove said oxide oroxynitride containing film from said surface of said silicon nitridelayer, andThe gas composition includes as follows:

    ______________________________________                                        0-90%              HBr                                                        9-100%             SF.sub.6.                                                  0%                 oxygen or oxidant gas.                                     Reactor conditions                                                            pressure           low                                                        Temperature        25 degrees C.                                              RF power           200-300 watts                                              rotational magnetic field                                                                        0-45 Gauss                                                 Process                                                                       Gass Flow, sccm   Parameters                                                  HBr               0 to 20 sccm, 0-91%                                         SF.sub.6          2 to 10 sccm, 9-100%                                        Pressure, mT      20 to 150 milliTorr                                         Power Density     0.82 to 3.3 W/cm.sup.2                                      Time, sec         4 to 20 sec.                                                Magnetic Field    0 to 45 Gauss                                               ______________________________________                                    

(2) a main step of etching the newly exposed surface of said siliconnitride layer with etchant gases having high selectivity with respect tosaid silicon oxide layer underlying said silicon nitride layer employsas follows:

    ______________________________________                                        Process            Parameters                                                 ______________________________________                                        Gas Flow, sccm                                                                HBr                10 to 30 sccm, 71-96%                                      O.sub.2            0.15 to 2 sccm, 2-7%                                       He                 0 to 10 sccm, 0-25%                                        Pressure, mT       20 to 150 milliTorr                                        Power Density      0.82 to 3.3 W/cm.sup.2                                     Etch Rate          300 to 1000 A/min                                          Magnetic Field     0 to 75 Gauss                                              Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                    LPCVD Nitride      >4:1                                                       Plasma Nitride     >4:1.                                                      ______________________________________                                    


2. A method of etching a silicon nitride layer on a substrate, inaccordance with claim 1 comprising as follows:(1) Breakthrough stepprocess materials and parameters are employed as follows:

    ______________________________________                                        Process           Parameters                                                  ______________________________________                                        Gas Flow, sccm                                                                HBr               5 to 15 sccm, 45-83%                                        SF.sub.6          3 to 6 sccm, 17-55%                                         Pressure, mT      50 to 100 milliTorr                                         Power Density     1.63 to 2.4 W/cm.sup.2                                      Time, sec         6 to 10 sec.                                                Magnetic Field    0 to 25 Gauss                                               ______________________________________                                    

(2) the main step of etching has parameters as follows:

    ______________________________________                                        Process            Parameters                                                 ______________________________________                                        Gas Flow, sccm                                                                HBr                15 to 25 sccm, 80-95%                                      O.sub.2            0.3 to 1 sccm, 2-5.7%                                      He                 0 to 2 sccm, 0-20%                                         Pressure, mT       50 to 120 milliTorr                                        Power Density      1.63 to 2.4 W/cm.sup.2                                     Etch Rate          500 to 700 A/min                                           Magnetic Field     25 to 60 Gauss                                             Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                    LPCVD Nitride      >6:1                                                       Plasma Nitride     >6:1                                                       ______________________________________                                    


3. A method of etching a silicon nitride layer on a substrate, inaccordance with claim 2 comprising as follows:(1) Breakthrough stepprocess materials and parameters are as follows:

    ______________________________________                                        Process              Parameters                                               ______________________________________                                        Gas Flow, sccm                                                                HBr                  10 sccm, 77%                                             SF.sub.6             3 sccm, 23%                                              Pressure, mT         50 milliTorr                                             Power Density        1.65 W/cm.sup.2                                          Time, sec            8 sec.                                                   Magnetic Field       0 Gauss                                                  ______________________________________                                    

(2) the main step of etching has parameters as follows:

    ______________________________________                                        Process               Parameters                                              ______________________________________                                        Gas Flow, sccm                                                                HBr                   20 sccm, 93%                                            O.sub.2               0.45 sccm, 2.1%                                         He                    1.05 sccm, 4.9%                                         Pressure, mT          100 milliTorr                                           Power Density         1.65 W/cm.sup.2                                         Etch Rate             600 A/min                                               Magnetic Field        45 Gauss                                                Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                    LPCVD Nitride         12.6:1                                                  Plasma Nitride        13.5:1                                                  ______________________________________                                    


4. A method of etching a silicon nitride layer on a substrate, saidsubstrate coated on its surface with a silicon oxide layer, in turncoated on its surface with a silicon nitride layer, in turn coated onits surface with an oxide or oxynitride containing film, said methodcomprising performing in a plasma reactor the steps, as follows:(1) abreakthrough step of employing a plasma of oxygen free etchant gases tobreak through and to remove said oxide or oxynitride containing filmfrom said surface of said silicon nitride layer, under the conditions asfollows:

    ______________________________________                                        Process           Parameters                                                  ______________________________________                                        Gas Flow, sccm                                                                HBr               0 to 20 sccm, 0-91%                                         SF.sub.6          2 to 10 sccm, 9-100%                                        Pressure, mT      20 to 150 milliTorr                                         Power Density     0.82 to 3.3 W/cm.sup.2                                      Time, sec         4 to 20 sec.                                                Magnetic Field    0 to 45 Gauss                                               ______________________________________                                    

(2) a main step of etching the newly exposed surface of said siliconnitride layer with etchant gases having high selectivity with respect tosaid silicon oxide layer underlying said silicon nitride layer employsas follows:

    ______________________________________                                        Process            Parameters                                                 ______________________________________                                        Gas Flow, sccm                                                                HBr                10 to 30 sccm, 50-90%                                      O.sub.2            0.15 to 2 sccm, 2-15%                                      He                 0 to 10 sccm, 0-30%                                        SiF.sub.4          0.5 to 2.5 sccm, 5-15%                                     Pressure, mT       20 to 150 milliTorr                                        Power Density      0.82 to 3.3 W/cm.sup.2                                     Etch Rate          200 to 700 A/min                                           Magnetic Field     0 to 75 Gauss                                              Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                                       >6:1                                                       LPCVD Nitride                                                                 ______________________________________                                    


5. A method of etching a silicon nitride layer on a substrate, inaccordance with claim 4 comprising as follows:(1) Breakthrough stepprocess materials and parameters are employed as follows:

    ______________________________________                                        Process           Parameters                                                  ______________________________________                                        Gas Flow, sccm                                                                HBr               5 to 15 sccm, 45-83%                                        SF.sub.6          3 to 6 sccm, 17-55%                                         Pressure, mT      50 to 100 milliTorr                                         Power Density     1.63 to 2.4 W/cm.sup.2                                      Time, sec         6 to 10 sec.                                                Magnetic Field    0 to 25 Gauss                                               ______________________________________                                    

(2) the main step of etching has parameters as follows:

    ______________________________________                                        Process            Parameters                                                 ______________________________________                                        Gas Flow, sccm                                                                HBr                11 to 20 sccm, 70-85%                                      O.sub.2            0.3 to 1 sccm, 3-7%                                        He                 0 to 2 sccm, 0-20%                                         SiF.sub.4          1 to 2 sccm, 5-9%                                          Pressure, mT       50 to 120 milliTorr                                        Power Density      1.63 to 2.4 W/cm.sup.2                                     Etch Rate          250 to 450 A/min                                           Magnetic Field     25 to 60 Gauss                                             Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                                       >10:1                                                      LPCVD Nitride                                                                 ______________________________________                                    


6. A method of etching a silicon nitride layer on a substrate, inaccordance with claim 5 comprising as follows:(1) Breakthrough stepprocess materials and parameters are employed as follows:

    ______________________________________                                        Process              Parameters                                               ______________________________________                                        Gas Flow, sccm                                                                HBr                  10 sccm, 45-83%                                          SF.sub.6             3 sccm, 17-55%                                           Pressure, mT         50 milliTorr                                             Power Density        1.65 W/cm.sup.2                                          Time, sec            8 sec.                                                   Magnetic Field       0 Gauss                                                  ______________________________________                                    

(2) the main step of etching has parameters as follows:

    ______________________________________                                        Process               Parameters                                              ______________________________________                                        Gas Flow, sccm                                                                HBr                   15 sccm, 83%                                            O.sub.2               0.45 sccm, 2.6%                                         He                    1.05 sccm, 6.0%                                         SiF.sub.4             1.5 sccm, 8.4%                                          Pressure, mT          100 milliTorr                                           Power Density         1.65 W/cm.sup.2                                         Etch Rate             300 A/min                                               Magnetic Field        45 Gauss                                                Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                                          >17:1                                                   LPCVD Nitride                                                                 ______________________________________                                    


7. A method of etching a silicon nitride layer on a substrate, saidsubstrate coated on its surface with a silicon oxide layer, in turncoated on its surface with a silicon nitride layer, in turn coated onits surface with an oxide or oxynitride containing film, said methodcomprising performing in a plasma reactor the steps on said substrate,as follows:(1) a breakthrough step of employing a plasma of oxygen freeetchant gases to break through and to remove said oxide or oxynitridecontaining film from said surface of said silicon nitride layer, andThegas composition includes

    ______________________________________                                        0-91%              HBr                                                        10-100%            fluorine-containing gas.                                   0%                 oxygen or oxidant gas.                                     reactor conditions                                                            pressure           low                                                        Temperature        25 degrees C.                                              RF power           200-300 watts                                              rotational magnetic field                                                                        0-45 Gauss                                                 Process           Parameters                                                  ______________________________________                                        Gas Flow, sccm                                                                HBr               0 to 20 sccm, 0-91%                                         SF.sub.6          2 to 10 sccm, 9-100%                                        Pressure, mT      20 to 150 milliTorr                                         Power Density     0.82 to 3.3 W/cm.sup.2                                      Time, sec         4 to 20 sec.                                                Magnetic Field    0 to 45 Gauss                                               ______________________________________                                    

(2) a main step of etching the newly exposed surface of said siliconnitride layer with etchant gases having high selectivity with respect tosaid silicon oxide layer underlying said silicon nitride layer employsas follows:

    ______________________________________                                        Process            Parameters                                                 ______________________________________                                        Gas Flow, sccm                                                                HBr                10 to 30 sccm, 71-96%                                      O.sub.2            0.15 to 2 sccm, 2-7%                                       He                 0 to 10 sccm, 0-25%                                        Pressure, mT       20 to 150 milliTorr                                        Power Density      0.82 to 3.3 W/cm.sup.2                                     Etch Rate          300 to 1000 A/min                                          Magnetic Field     0 to 75 Gauss                                              Si.sub.3 N.sub.4 -to-SiO.sub.2 selectivity                                    LPCVD Nitride      >4:1                                                       Plasma Nitride     >4:1                                                       ______________________________________                                    